Pressure usually only creates stress in the pipe, rather than loadings on supports/restraints, because pressure loads are neutralized at the cross-section by the tension in the pipe wall. One exception to this is when the pipe is not continuous from anchor to anchor, such that tension is not present in the pipe wall at all locations of the system. (Note that a second exception occurs when the Bourdon effects of pressure are considered. The Bourdon effect is due to the axial extension of pipes either under high pressure or in long runs, causing displacements which must be absorbed by the piping system. Since this is a displacement load, it is a secondary load, and therefore is not considered here.)
Tension in the pipe wall is not continuous when there are expansion joints or slip joints present in the system. These types of components are too flexible in the axial direction to transmit the pressure force, therefore the unbalanced pressure load must be handled by either tie rods or external pipe restraints. The unbalanced pressure load is calculated as:
When using restraints to absorb the unbalanced pressure load, it is recommended that guides be located on the adjacent pipe runs in order to reduce the danger of buckling. The Expansion Joint Manufacturers Association recommends that the first guide be placed a distance no further than 4 pipe diameters from the expansion joint, with the second guide placed no further than 14 pipe diameters from the first.
Figure 2-3 shows some typical piping layouts using expansion joints.
More information on the use of expansion joints is found in Section 2.3.6 (Stress Reduction through Use of Expansion Joints) and Section 3 (Modeling And Analysis Of The Piping System) of these Pipe Stress Analysis notes.
To get our piping stress analysis services, please contact our piping engineering firm for a free quote.